Low damping factor but fast& high current SS amps?

Zuio - your speaker, most likely, has inductor in series with the woofer. This inductor commonly has resistance of 0.08ohm (or more) making DF<100 no matter what amplifier you connect to it. Look in the middle of the table of the best solid core inductor I could find in Partsexpress:

http://www.parts-express.com/14-gauge-c-core-toroidal-inductors.cfm

"So, I believe that high DF indirectly let's us know that the amp is trending towards high bandwidth (fast) & will be capable of high current delivery (if it's power supply is able & the music signal demands it)."

High current demand is at lower frequencies where amplifier's output impedance is usually very low. High DF doesn't tell us anything about amplifier's bandwidth. Amplifier might have bandwidth limited at the input (to avoid TIM) while output is wide bandwidth with low output impedance. In addition output impedance might or might not change a lot with frequency. For instance Rowland 625 has DF=200@20Hz-20kHz while my class D Rowland 102 has DF=8000@20Hz and DF=8@20kHz.

Magfan - Deep feedback lowers output impedance (also lowers THD and IMD). Amplifier's configuration affects initial output impedance. Class D amps for instance connect speaker to power supply and GND (zero impedance points) all the time, only polarity changes. Without feedback output impedance would consist of Mosfets' resistance (in order of 0.1ohm) and zobel network's impedance (common mode choke + capacitor and resistor to filter out carrier). Inductance of this choke is the reason of class D higher output impedance at high frequencies. On the other hand this choke has only few turns of wire and very low resistance (high DF at low freq.). I would estimate that since 0.1ohm is reduced to 0.001ohm feedback is at least 40dB deep but it is not too bad.

There is nothing wrong with feedback, if you know how to use it. Amplifier has to be as linear and fast as posible to start with. Small amount of feedback should reduce THD only to about 0.2-0.5% while bandwidth at the input should be limited to one that amp had without feedback (to prevent TIM).

I'm trying to make sens of it and so far I've learned that things are extremely complicated. Very high DF means deep feedback but Soulution 700 amp has DF=10000 and bandwidth of 1MHz with no feedback. If I would only know how to do it I would build such amps and sell them (for $100k) myself.

"How does feedback lower DF"

Magfan – It increases DF (lowers output impedance).
Let’s take amplifier that has gain of 30 (31.6dB). When input voltage is 1V output voltage is 30V. Output voltage drops (for whatever reason) 1V under 1A load to 29V. That's 1ohm output impedance (DF=8).

Now, let's build this amp with gain of 300 but feed 3% of the output voltage back to the input in opposite phase. As a result amplifier’s output is the same 30V as before but input is the difference between 1V and 3% of 30V = 0.1V Let’s verify (1V-0.03*30V)*300=30V

Let’s load this amplifier with 1A. Our voltage drop inside is still 1V under 1A load, but output voltage will be higher than 29V because we subtract less from the input. Output voltage will be 29.9V and output impedance will be 0.1V/1A=0.1ohm (DF=80). Let’s verify. (1V-0.03*29.9V)*300-1Vdrop=29.9V.

Output impedance dropped 10 times. Expression 1+B*Aol is known as “Improvement Factor”. In our case B (“Feedback Factor”) = 0.03 (3%), Aol (“Open Loop Gain”) = 300 thus Improvement Factor = 1+0.03*300=10.

It is a little clumsy, but I didn’t want to bring whole feedback theory equations into simple example.

Liquy - I never said they are related. That wasn't my quote.

Negative feedback increases both DF and the bandwidth but it does not mean that high DF = fast amplifier. Amplifier might be slow to start with or have bandwidth intentionally limited at the input.

Slew rate might be important but let's see how much do we need. If music calls for 3V swing at 20kHz it corresponds to 3V*2pi*20kHz=376800V/s=0.38V/us and it is very slow.

It is hard to predict speed from DF. For instance Atmasphere S30 amplifier has extremely low DF=1 and extremely high slew rate of 600V/us.